Linking phyotplankton and bacterioplankton community dynamics to iron-binding ligand production in a microcosm experiment

Organic ligands shape the marine biogeochemical cycle of Fe by controlling the solubility of Fe in the ocean. The strongest of detectable marine ligands are presumed to be biological in origin, but definitive functional linkages between specific marine microbes and strong Fe-binding ligands produced in situ remain largely unknown. Here we present evidence that a handful of copiotroph bacterial taxa are robustly associated with the strongest fraction of Fe-binding ligands detected at the initial collapse of a microcosm phytoplankton bloom. Metabolically active but episodically abundant copiotrophic bacterial taxa are known to rapidly remineralize phytoplankton blooms, and we additionally suggest that during initial phases of biomass remineralization they produce strong Fe-binding ligands to extract biogenic Fe. Our results could provide a mechanistic explanation for the apparently paradoxical strong ligand spikes detected after Fe fertilization in field studies and suggests that strong ligand production may be a control on overall Fe recycling efficiency.